AuthorTopic: VPR - Effective VLF absorber? (Read 8649 times)

Please find below the link to a document entitled "Novel silencers and absorbers for wind tunnels and acoustic test cells". It is based on research by Fraunhofer-Institut für Bauphysik (IBP) and details an absorber which has significant absorption well below 80 Hz while being just over 4" deep. Seems too good to be true, but I am optimistic. A member on another forum has built something inspired by this, in a typical small room, and it seems to work miraculously well... but as testing is not complete, details and confirmations are not up yet. The thing that is particularly exciting is that this doesn't seem to be a narrow 'q' absorber like many helmholtz and panel type devices... its being called a 'shelving' absorber... like your "broadband" porous absorber, only in a much lower frequency range.

Does anybody here have experience with these type of traps? What are the things to be wary of, if attempting to DIY?

There are clear indicators that a rust resistant, powder coated steel plate that is 60" x 40" and backed by a porous absorber that has a density of 40 Kg/m3 is effective down to 35 Hz... The placement of the absorber will be critical... but what else?

I am aware of the modex stuff from RPG built on these principles, but I am enthusiastic about the prospect of a DIY solution. Since these documents are floating around in the public domain, I suppose it would be alright to build these for non commercial purposes...?

I haven't dealt with these much. I've heard of them a lot lately and studied them a bit. I'm always uncomfortable with things I haven't had hands on & repeated experience with. But I'm curious.

The thing I'm always careful with is the differences in the quality benchmarks we set between industrial projects and Professional Studio Design projects.

If it works in an industrial environment, it is definitely worth giving it a try in a studio - and this is how I usually filter "acoustic" products. Not used in the heavy industrial systems: not interested. But keep in mind that the standards in the industrial world are often rather loose compared to the standards we set for studios, so this can only be an indication that it has a potential in studios.

I think you can definitely try to DIY one of these. It's not complicated. But you just can't try it on a 'real' project, too risky. You have to try it for yourself in a known environment, which could still be rather expensive even if DIY. But it's the only way to get hands-on experience.

We have an approach that is a *bit* similar using Polymethyl methacrylate (PMMA) as it has quite a particular behavior & resilience to start with, in a constraint layer using air as a spring and by managing the pressure before and after the membrane (this is how it is constraint) - that system being usually part of a larger system also using resistance to flow to take care of the frequencies higher than 100Hz. These membranes are usually pretty big and cover a substantial amount of surface, sometimes whole walls. The space behind them is fully sealed.

Which results in a very wide band trap, usually efficient over the whole spectrum, with an emphasis on frequencies under 200Hz.

If you do try these VPR systems, I'll be very curious to see what the results and your opinion will be.